The invention relates to a surface-alloyed cylindrical, partly cylindrical or hollow cylindrical structural member, consisting of an aluminium matrix casting alloy and a precipitation zone extending as far as the surface of the structural member, comprising an aluminium based alloy with precipitated hard phases.
A method for coating inner bearing surfaces of cylinder walls is disclosed in DE-OS 198 17 091. According to claim 1 of the Offenlegungsschrift, wear-resistant surfaces are produced on the inner piston bearing surfaces of light metal engine blocks by using a probe for the continuous supply of silicon powder in which an energy beam with a radiation spot travelling spirally over the surface is moved relative to the light metal engine block held in a fixed position. At a laser light power of approximately 2 kW and a radiation spot diameter of approximately 0.5 to 2 mm approximately 10 g of powder per minute is applied to the surface and alloyed. By this means a hard material component of 20 to 50% can be alloyed into the surface at a penetration depth of approximately 1 mm.
Attempts were also made to develop a method for manufacturing a light metal cylinder block in which a laser beam having a strip width of at least 2 mm transverse to the feed direction is guided over the light metal matrix surface held in a fixed position. In this case the powder is first heated to the melting point at the point of incidence of the laser beam on the light metal matrix surface and then diffuses therein. Primary silicon is formed in the alloying zone where an average layer thickness of 150 to 650 xcexcm is given as preferred in the matrix alloy. The laser light power is preferably 3 to 4 kW where linear focussing systems can also be used.
The structure attainable by the afore-mentioned method consists of a light metal matrix alloy with a finely disperse surface layer containing primary silicon precipitations which exhibits rounded grains having an average grain diameter between 1 and 10 xcexcm. In addition to the pure aluminium phase, the surface layer also contains 10 to 14% AlSi eutectic and 5 to 20% primary silicon where the minimum hardness is approximately 160 HV.
Structural members insensitive to thermal shock are required for specific applications. This can only be achieved using methods of manufacture known so far by using very expensive treatment measures. A characteristic feature for these heavy-duty structural members is a slow increase in hardness from the matrix as far as the surface layer where the total increase in hardness should extend over a range of 200% relative to the initial hardness of the matrix alloy.
The object of the present invention is to develop tribologically optimised, heat-treatable cylinder blanks having new micro-structural properties and near-surface material changes. The new micro-structural properties and near-surface material changes should in particular make it possible to use the casting blank as structural members exposed to vibrations, such as, for example, bearing surfaces of bearing boxes of reciprocating machines, wear surfaces of brake parts, and sliding and friction surfaces of all types.
This object is solved according to the invention by the features specified in the claims. Using the structure according to the invention comprising matrix, precipitation zone and an eutectic transition zone supersaturated by primary hard phases, it is possible to manufacture various structural members having wear surfaces such as, for example, sliding surfaces (crankshaft bearings), friction surfaces (brake discs) as well as engine blocks and cylinder heads, including the valve seat rings located therein, having especially favourable properties. The properties are characterised by a slow increase in hardness from the matrix as far as the surface layer. These properties predestine structural members thus equipped for applications where structural members insensitive to thermal shock are required.
Different types of alloys can be used in a coating layer. The powder can be applied to the surface of the workpiece in a single stage (single powder irradiation) or in several stages (several powder irradiations) via suitably shaped powder slit nozzles. The linear focal width is at least 4 mm, preferably 5 to 15 mm.
The alloys AlSi and AlSiCu as well as AlSiCuNi and Mg-containing Al alloys are suitable as the workpiece for single-stage powder supply where silicon is fed in the powder jet. For a two-stage powder supply lead as well as silicon could also be applied as powder material to an aluminium-silicon casting blank.